In helicopter borne air-sea rescue operations, radio-equipped divers are frequently dropped into the water from a substantial height. For a diver to reach a depth of 3 meters below the surface of the water upon such an entry is a possible experience, and in exceptional cases depths of as much as 10 meters may be momentarily attained.
It is desirable that the earphone of the two-way radio communication unit be configured to be worn in the ear. Furthermore, any such earphone must be able to withstand at least momentary immersion, preferably to as much as 10 meters, and upon returning to the surface be immediately in serviceable condition. This requires that an in-the-ear earphone must be sealed against water entry, and must also provide adequate signal output once the earphone is no longer immersed. This requires some form of water entry barrier system to the interior of the earphone.
One approach to forming a barrier which is water-impermeable but sound permeable is disclosed in U.S. Pat. No. 4,987,597, issued to Haertl, Jan. 22, 1991. As disclosed therein, a membrane seal made of porous hydrophobic polytetrafluoroethylene is disposed to sealingly cover the output conduit of an in-the-ear hearing aid. The purpose of the membrane is to allow sound to pass through, but to reject any entry of perspiration. The hydrophobic property of the membrane prevents water from entering the pores of the structure; however, in the absence of water blockage, the membrane can successfully pass sound.
Attempts were made to employ this principle to a deep-immersion microphone. Similar membranes were affixed to the outlet conduits of in-the-ear hearing aid transducers. It was soon discovered that such a membrane, when made sufficiently stiff to successfully resist water at 10 meter immersion pressures, introduced an unacceptable degree of sound absorption. Alternative approaches using various forms of sealing diaphragms, either alone or in combination, resulted in structures that were either physically too large or insufficiently sound transmissive.
The present invention is oriented toward a solution of these and other problems.